This invention relates to a novel crystal modification of a fluoran compound, a process for its preparation and recording materials that use it.
More specifically, this invention relates to a novel crystal modification of a fluoran compound that is useful as an electron donating color former in recording materials such as a pressure-sensitive recording material and a thermal recording material. The invention also relates to a process for preparing said novel crystal modification, as well as recording materials that contain said crystal modification and which exhibit superior characteristics.
Recording materials that make use of the color forming reaction between colorless or pale-colored electron donating color formers (hereunder referred to simply as "color formers") and organic or inorganic electron accepting color developers (hereunder referred to simply as "color developers") are well known and classified as pressure-sensitive recording materials, thermal recording materials, electrothermal recording materials, etc. Pressure-sensitive recording materials which are typically described in Japanese Patent Publication No. 20144/1967, etc. are used in such fields as vouchers and printers on computers. Thermal recording materials which are typically described in Japanese Patent Publication No. 14039/1970, etc. have a broad range of applications including recorders for instrumentation, facsimile, printers and automatic ticket vending machines.
Such recording materials are required to exhibit high performance in various respects including whiteness of background, storage stability of the background, color rendition, sensitivity for color formation, initial color density and storage stability of a color image, and it is essential to use color formers that specifically suit these purposes. Color formers for use in pressure-sensitive recording materials must satisfy an additional important characteristic requirement for high solubility in inner-phase solvent(s) (encapusulation oil(s)). Fluoran compounds are extensively used as color formers in various type of conventional recording materials.
While many compounds have already been known as fluoran compounds, one which has the same structural chemical formula as the fluoran compound according to the present invention [formula (1)] is disclosed in Japanese Patent Publication No. 17490/1974 and Japanese Patent Public Disclosure (Laid-Open) No. 34422/1972, which state that the compound has a melting point in the neighborhood of 158.degree.-165.degree. C. (this known fluoran compound is hereunder referred to as the ".alpha.-type crystal modification"). Pressure-sensitive recording materials using this .alpha.-type crystal modification exhibit low whiteness and lightfastness of the CB surface and the color image formed on those materials is also low in lightfastness. Thermal recording materials using this .alpha.-type crystal modification are low not only in background whiteness but also in the fastness of the color image formed on those materials. ##STR1##
Fluoran compounds of formulas (2) and (3) are also known to have chemical structures similar to that of the fluoran compound according to the present invention and they are described in Japanese Patent Publication Nos. 32767/1974 and 17490/1974: ##STR2##
The fluoran of formula (2), however, has the problem that it has too low a solubility in inner-phase solvent(s) to be effectively used as a color former in pressure-sensitive recording materials. Pressure-sensitive recording materials using this fluoran exhibit low whiteness and lightfastness of the CB surface and the color image formed on those materials is also low in lightfastness. Thermal recording materials using the fluoran (2) are low not only in the lightfastness of the background but also in the dynamic sensitivity for color formation and the storage stability of the color image formed on those materials. As a further problem, if the fluoran under consideration is used in common pressure-sensitive recording materials which use a zinc salt of a salicylic acid derivative as a color developer (which recording materials are hereunder referred to as "conventional pressure-sensitive recording materials") and in common thermal recording materials which use bisphenol A as a color developer (which recording materials are hereunder referred to as "conventional thermal recording materials"), the color rendered is purplish black and additional toning is necessary to attain a pure black color.
The fluoran of formula (3) has the problem that it has too low a solubility in inner-phase solvents to be effectively used as a color former in pressure-sensitive recording materials. Pressure-sensitive recording materials using this fluoran exhibit not only low whiteness of the CB surface but also insufficient lightfastness; in addition, the color image formed on those materials is low in lightfastness. Thermal recording materials using the fluoran (3) exhibit not only low whiteness and lightfastness of the background but also poor dynamic sensitivity for color formation; in addition, the storage stability of the color image formed on those materials is by no means sufficient. As a further problem, if the fluoran under consideration is used in conventional pressure-sensitive and thermal recording materials, the color rendered is greenish black and additional toning is necessary to attain a pure black color.